1. Field of the Invention
The present invention relates to a band pass filter which is preferably applied to, for example, a radio apparatus used in an earth station for satellite communication. Further, the band pass filter can be a type with a variable center frequency.
In a radio apparatus used in an earth station for satellite communication, it is a recent trend to enable the center frequency of the band pass filter (hereinafter referred to simply as BPF), which is located at a stage after a frequency conversion stage in the apparatus, variable, in order to make the local oscillator in the apparatus operate as a synthesizer. This is because, the frequency allocation for each earth station is often changed for a variety of reasons. Therefore, it is desired for each earth station to have a variable frequency local oscillator. For this, the BPF should accordingly also be a variable center frequency type.
In the prior art, as will be explained hereinafter in detail, the variable center frequency BPF produces the following two disadvantages. The first is that the BPF becomes relatively large in size. The second is that insertion loss by the insertion of a center frequency varying means into the BPF is increased. This causes a undesired reduction of attenuation level in a frequency range outside the frequency range to be passed through the BPF and also undesired distortion of the filtering characteristics.